1. Field of the Invention
The present invention relates to a sizing composition for glass strands, this composition reacting to heat. The present invention also relates to a process for the production of reinforcing glass strands using this composition as well as to the glass strands obtained and to the composites produced from the strands.
In the continuation of the text, "polymerization", "to polymerize", "polymerizable" and the like are understood to mean "polymerization and/or crosslinking", "to polymerize and/or to crosslink", "polymerizable and/or crosslinkable" and the like respectively.
2. Description of the Background
Reinforcing glass strands are manufactured in a known way from molten glass streams flowing from die orifices. These streams are drawn in the form of continuous filaments and then these filaments are gathered into base strands, which are then collected.
Before they are gathered together in the form of strands, the filaments are coated with a size by passing over a sizer device. This deposition is necessary for the production of the strands and makes it possible to combine them with other organic and/or inorganic materials to produce composites.
The size serves, in the first place, as lubricant and protects the strands from the abrasion resulting from the high-speed friction of the strands over various devices during the abovementioned process.
The size can also ensure, in particular after polymerization, the integrity of the abovementioned strands, that is to say the binding of the filaments to one another within the strands. This integrity is in particular sought after in textile applications where the strands are subjected to strong mechanical stresses. Indeed, if the filaments show little attachment to one another, they break more readily and disrupt the operation of the textile machinery. Strands that do not exhibit integrity are moreover regarded as difficult to handle.
The size also facilitates the wetting and/or the impregnation of the strands by materials to be reinforced and helps in the creation of bonds between the said strands and the said materials. The mechanical properties of the composites obtained from the material and the strands depend in particular on the quality of the adhesion of the said material to the said strands and on the ability of the said strands to be wetted and/or to be impregnated by the said material.
The sizing compositions used must be sufficiently stable and compatible with the drawing rates of the filaments which have to pass through them (several tens of meters per second). They must in particular resist the shearing induced by the passage of the filaments and properly wet their surface at the said rates. In the case where they thermally polymerize, they must exhibit a reaction temperature which is sufficiently high to remain stable at the die. It is also desirable for these compositions to exhibit, after polymerization, a maximum degree of conversion (this degree corresponding to the ratio of the level of functional groups which have reacted in the size after heat treatment to the level, in the size, before heat treatment of reactive functional groups capable of reacting), in order to guarantee in particular that sized strands of constant quality are obtained (a size exhibiting a degree of conversion which is much below the theoretical degree expected being capable of changing overtime).
The majority of sizes currently used are aqueous sizes which are easy to handle but which have to be deposited in large amounts on the filaments in order to be effective. The water generally represents 90% by weight of these sizes (in particular for reasons of viscosity), which necessitates the drying of the strands before they are used as reinforcers, it being possible for the water to be harmful to the good adhesion between the strands and the materials to be reinforced. These drying operations are lengthy and expensive, must be adapted to the manufacturing conditions of the strands and their efficiency is not always optimum. When they are carried out during the fibre-drawing operation (that is to say before collecting the strands obtained by gathering together the filaments), on the filaments (WO 92/05122) or on the strands (U.S. Pat. No. 3,853,605), they require the installation of drying devices at each die and when they are carried out on the strand wound packages, they bring about risks of uneven and/or selective migration of the components of the size within the wound packages (aqueous sizes already having a tendency to spread themselves unevenly over the strands because of their nature) and optionally phenomena of colouring of the strands or of deformation of the wound packages. The deformation of the wound packages is also observed, in the absence of drying, on straight-edged wound packages (rovings) of fine strands (that is to say exhibiting a "count" or "mass per unit length" of 300-600 tex (g/km) or less) coated with aqueous sizes.
A few exceptional patents describe non-aqueous sizes but these sizes generally involve organic solvents which are problematic to handle and which can, because of their toxicity, harm the health of people in the vicinity and/or pose problems of viscosity which it is appropriate to solve by heating these sizes (U.S. Pat. No. 4,604,325) or by adding suitable agents (U.S. Pat. No. 4,609,591). These sizes also often require the installation of specific devices below each die; it is in particular necessary, when the strands are collected in the form of wound packages, to treat the strands before obtaining the wound packages in order to prevent the turns of each wound package from sticking to one another, this sticking phenomenon making it difficult to unwind the strands. These treatments, the effectiveness of which depends on the operating conditions, consist, for example, in polymerizing the size by subjecting the sized strands to the effect of ultraviolet radiation in order to confer on them a satisfactory integrity and to make it possible to handle them (U.S. Pat. No. 5,049,407). The polymerized size, however, prevents the filaments from sliding with respect to one another, this absence of mobility resulting in the shattering, by mechanical degradation of the size, of the strands when they are cut and it being possible for this absence of mobility to cause problems in textile applications where the strands used must both exhibiyt integrity and be flexible.